I think it's a lot of what others have said - PL pages taking over from layouts on blogs and in galleries, plus a lot of it all starts to look the same if people are posting every week, and I think some people are just sick of hearing about it if they've decided not to do it. I would also say that some people misunderstand what PL is or can be and they reject the idea of scrapping their everyday "boring" lives or they think it's all about a picture-a-day project and they dislike that idea (without necessarily realizing it doesn't have to be that at all).
But I think some of it is the relentless marketing of the BH company. I'm really kind of sick of BH at this point... and I hate saying that! But I am. When it was only a couple of core kits each year it was one thing - I was obsessed with the idea behind it. But now there are dozens, and an app, and partnerships, and exclusives at certain stores, and it all seems a bit too much. And I personally got to a point where I didn't need to hear her speak ever again - I knew no matter what the discussion was about, she would be talking about this system in a very specific "marketing" kind of way and I don't need to hear that any more. (I mean, I get it! because it's her business, and people who love it, love it. But... sigh. It got to be tiresome, for me, who DOES love PL.)
And then everyone else is producing pocket page stuff too. So if you weren't a fan to start with, I could definitely see it getting the point where you just never want to hear the words Project Life again. (That's not me, but I get it).